Matter of Sorting element - What is Custom Sort?
Custom Sort: In general, you can sort a list of objects by key. E.g. if you have a list of customers, you can sort them by various keys such as first name, last name, customer number, etc. 

Key - a key for a custom sort, which can be written as an expression using the attributes of your list element e. For example, you can enter e.location.x as as a Key to sort the list of objects based on their x location.

Key List - list of sorting keys. If your sorting key is not an attribute of the objects in the list, you can plug the list of key into the Key List Node. For example, look at this answer, where the polygons are sorted according to the distance to some point using Sort List node. Computed distances are used as a Key List.